Generally, sensors are used in the gathering of information for monitoring and controlling systems. Information gathered by sensors is subsequently manipulated by information processing systems in such a way that it becomes accessible to humans or to control the information state directly. In such systems, the sensor plays a central role as it creates input data that is subsequently processed. Sensors take on many shapes and forms having varied and unique operating requirements. For example, depending on the sensor and application, the sensor may require a bias voltage to operate. The bias voltage may be used either to stimulate the sensor to perform its task (e.g., stimulating a transducer) or as a reference voltage used to determine a change in the sensor's environment.
In the specific case of bias voltage as employed with electrochemical gas sensors, however, the purpose is to establish the operating characteristics of the electrochemical cell. In other words the sensitivity, stability, specificity, life, noise-level, etc. to the target gas are determined in-part by the bias voltage. Thus, the bias voltage is one of the design parameters considered in the development of a gas detecting electrochemical cell. For example, for a cell with a specific chemistry, the bias voltage may determine to which, of a variety of gases, the cell will be most sensitive.
As an example, when used in gas detection applications, most electrochemical sensors require an externally generated bias voltage to be applied between sensor electrodes. Electrochemical gas sensors can indicate the presence of gas by either a change of potential across the sensor's electrodes or by causing a current to flow through an external circuit. Depending on the sensor type and application, the "bias" voltage may be zero, positive, or negative. The stability of the bias voltage is important, however, because a fluctuation of the voltage can cause the sensor to respond as though it had detected gas. Also, errors in the bias voltage value can change the sensor's detection characteristics as described above.
Manufacturers of information detection systems have previously utilized various circuit techniques to produce sensor bias voltages. For example such techniques have included the selection of resistors having discrete resistance values or adjusting potentiometers to desired resistance levels that are used in voltage divider networks to produce the required bias voltage. These practices, however, are labor-intensive and difficult to automate. Additionally, once implemented, such bias circuits are specific to the bias requirements of a particular sensor. In an effort to accommodate the bias needs of multiple sensors, resistor ladder circuits and digital to analog converters are utilized but are cumbersome, complex and expensive to implement.
Thus, there is a need for a sensor bias circuit that utilizes simple and inexpensive elements, provides a stable bias, and can be automatically programmed to accommodate various sensor bias requirements. The present invention provides such a bias circuit.